[3/N] WindowDecorViewHost: Migrate WindowDecoration's SCVH

Spiritual revert^2 of Iae7589879d56d802c85b8c4a72a8dcc9b20d6afb, but
broken down into smaller changes.

Replaces WindowDecoration's own ViewHost and sync/async relayout logic
to use WindowDecorViewHost/Supplier abstractions instead. This in
preparation for a follow up change to replace the Default*
implementation of WindowDecorViewHost/Supplier with an implementation
that reuses SurfaceControlViewHosts to reduce latency/jank.

This is also a pure refactor, so no behavior or performance changes are
expected with this CL alone.

Bug: 360452034
Flag: EXEMPT refactor
Test: m WMShellUnitTests
Change-Id: Iffc77bafaef5ccd7a353665bd87accdf1b5b4f44